Electrospray ionization (“ESI”) is an important technique for the analysis of biological materials contained in solution by mass spectrometry. See, e.g., Cole, R. B. Electrospray Ionization Mass Spectrometry: Fundamentals, Instrumentation & Applications; John Wiley and Sons, Inc.: New York, 1997. Electrospray ionization was developed in the late 1980s and was popularized by the work of Fenn. See, e.g., Fenn J B, Mann M, Meng C K, Wong S F & Whitehouse C M (1989) ELECTROSPRAY IONIZATION FOR MASS-SPECTROMETRY OF LARGE BIOMOLECULES. Science 246, 64-71. Simplistically, electrospray ionization involves the use of electric fields to disperse a sample solution into charge droplets. Through subsequent evaporation of the droplets, analyte ions contained in the droplet are either field emitted from the droplet surface or the ions are desolvated resulting in gas phase analyte ions. The source of the liquid exposed to the electric field and to be dispersed is ideally one of small areal extent as the size of the electrospray emitter directly influences the size of droplets produced. Smaller droplets desolvate more rapidly and have fewer molecules present per droplet leading to greater ionization efficiencies. These ions can be characterized by a mass analyzer to determine the mass-to-charge ratio. Further analyte structural information can be obtained by employing tandem mass spectrometry techniques.
The chemical informing power of electrospray ionization—mass spectrometry can be enhanced when the electrospray emitter is coupled to liquid-phase chemical separations such as liquid chromatography, capillary electrophoresis, or ion exchange chromatography, to name a few. These chemical separation techniques endeavor to deliver isolated compounds to the electrospray emitter to reduce ionization suppression and mass spectral complexity. More recently, chemical separation systems have been implemented on microfabricated fluidic devices that also have attempted to incorporate integrated electrospray emitters. See, e.g., Xue Q, Foret F, Dunayevskiy Y M, Zavracky P M, McGruer N E & Karger B L (1997), Multichannel Microchip Electrospray Mass Spectrometry. Anal Chem 69, 426-430, Ramsey R S & Ramsey J M (1997), Generating Electrospray from Microchip Devices Using Electroosmotic Pumping. Anal Chem 69, 1174-1178, Chambers A G, Mellors J S, Henley W H & Ramsey J M (2011), Monolithic Integration of Two-Dimensional Liquid Chromatography—Capillary Electrophoresis and Electrospray Ionization on a Microfluidic Device. Analytical Chemistry 83, 842-849.
Despite the foregoing, there remains a need for alternate microfluidic devices with ESI emitters that can be operated on a rapid time scale, increase ionization efficiency, and increase ion flux to enhance the performance of electrospray ionization—mass spectrometry analysis of chemical and biochemical materials.